This invention relates to semiconductor devices having well structures and more particularly to a method of selectively disordering regions or areas of such structures using laser annealing techniques.
The art of utilizing laser annealing as an effective means for treating semiconductor structures during their fabrication processes has mushroomed and has become extensive as an effective treatment before, during and after fabrication of semiconductor devices. Examples of such art are the conversion of predefined areas of amorphous or polycrystalline silicon or other semiconductor material into single crystal areas using laser beam annealing as exemplified in U.S. Pat. Nos. 4,330,363 and 4,388,145. Another example is U.S. Pat. No. 4,469,527 wherein a laser beam anneal is utilized to extinguish lattice defects in predefined areas of a substrate surface to render such areas semiconductive wherein the entire surface of the substrate had been previously rendered entirely semi-insulating through prior irradiation treatment.
In the early 1980's, techniques were developed for selective Zn diffusion of quantum well structures, e.g., having a single or multiple quantum well, comprised of semiconductor heterostructure materials, such as GaAs/GaAlAs, performed at temperatures (e.g., 600.degree. C.) below the epitaxial growth temperatures (e.g., 750.degree. C.) for such structures. See, for example, U.S. Pat. No. 4,378,255. The idea of a quantum well structure is to effect a basic improvement in the performance of such structures as utilized in various semiconductor devices, such as tunnel transistors, field effect transistors or semiconductor lasers, by confining carriers in a sufficiently narrow active region comprising a thin active well layer or layers to modify the band structure, thereby raising the lowest carrier states in the well or wells to a higher energy level. By utilizing thermal Zn diffusion, predefined areas of the quantum well structure may be disordered to form a disordered alloy that exhibits higher bandgap and lower refractive index properties compared to regions that are left undiffused. As a result, defined regions of higher bandgap can be created in a monolithic semiconductor structure or wafer useful in the fabrication of optical waveguides, current channels, semiconductor lasers, integrated optical electronic circuits and other types of integrated circuits.
More recently, it has been discovered that Si, Ge, Sn, K Al, S and other elements may be implanted and then annealed at temperatures at about 800.degree. C. or above to provide the same disordering.
The fundamental advantage in utilizing these diffusion and implantation techniques is the ability to epitaxially grow semiconductor devices comprising two dimensional layers that include a well structure, which may or may not exhibit quantum size effects, and thereafter converting predefined or selected areas of the well structure into disordered material to form three dimensional semiconductor devices.
An attractive and simpler method of achieving this goal would involve laser beam annealing of predefined or selective areas of well structures to bring about thermal disordering, vis a vis thermal diffusion or implant disordering. However, there are several problems that must be overcome to utilize such a technique. First, relative to the GaAs/GaAlAs regime, for example, the temperature of the laser beam must exceed the epigrowth temperature so that free interdiffusion of Ga and Al in the well structure may be thermally induced. The areas of the wafer scanned by the laser beam need to be heated to an elevated temperature in a range above the epitaxial growth temperatures in order to bring about enhanced or accelerated interdiffusion. Utilizing such high temperatures in predefined small areas of the wafer will produce large lateral temperature gradients which may damage the overall quality of the epitaxially deposited materials rendering the wafer useless for the fabrication of semiconductor devices. Second, in the GaAs/GaAlAs regime, the loss of As from the epitaxial grown wafer may be critical due to its volatility at such high temperatures thereby rendering the wafer useless for device purposes. Third, a significant temperature gradient between regions annealed compared to regions not annealed may cause the wafer to be stressed or cracked rendering the wafer structure useless for device purposes.
Thus, the utilization of laser beam annealing to accomplish thermal disordering in well structures in and of itself is not sufficient to accomplish a practical method or process that will achieve selective disordering in predefined areas of a semiconductor wafer having an epitaxially grown well structure.